We propose that the pathogenesis of Lyme arthritis is due to the persistence of Borrelia burgdorferi, and that B. burgdorferi with variable outer surface proteins (Osps) that do not bind protective antibody -- and therefore resist elimination by the host -- preferentially survive within infected joints. Our recent studies support this belief by showing that borreliacidal OspA or B antibodies elicited during infection in mice or humans are unable to clear the spirochete from the host although immunizations with murine or human OspA or B antibodies protect mice against infection. To determine if OspA or B from B. burgdorferi that survive within the synovium of patients with Lyme arthritis have variable OspA or B, we will characterize spirochete DNA recovered from clinical specimens. First, ospA/B will be amplified from affected synovial fluid at time points throughout the illness and analyzed for mutations. Second, OspA or B will be expressed in Escherichia coli and probed to determine whether protective OspA or B antibodies bind with the antigens. We predict, based on our preliminary data, that the proteins will not bind, or poorly bind, to these antibodies --suggesting that spirochetes expressing OspA or B that do not bind protective antibodies persist preferentially in infected patients. Moreover, as we and others have recently shown that OspC or F elicit protective responses (albeit less pronounced than OspA or B), we will also assess the variability of these proteins. Third, to determine whether patients have the capacity to mount a borreliacidal antibody response -- indicating that microbial persistence is not due to a host defect -- we will assess whether patient serum or synovial fluid is protective in mice and delineate the immune specificity. We will then determine how the selective pressure of protective antibody influences B. burgdorferi variability within a mixed population of spirochetes by allowing ticks infected with a heterogeneous population of B. burgdorferi to engorge upon mice and then comparing Osp variability of the spirochetes in the ticks, and B. burgdorferi recovered from infected mice. Our preliminary data show that OspA or B antibodies exert a selective pressure resulting in the enriched recovery of spirochetes with variant OspA or B from immunized mice -- analogous to our studies in patients. We will then assess Lyme disease in transgenic mice that are immunologically tolerant to specific B. burgdorferi Osps -- proposing that these mice will develop more severe infection than non transgenic litter mates because the selective pressure of Osp antibody has been ablated, and that variable B. burgdorferi will not preferentially persist in the transgenic mice. Our recent studies support this belief by showing that C3H mice infected with B. burgdorferi develop arthritis that subsides with the rise of high titers of B. burgdorferi antibodies, including OspA and B whereas scid develop persistent arthritis -- suggesting that the resolution of disease may be due to the establishment of a protective response. In summary, these studies will determine if protective antibodies, elicited during infection, cause the selective survival of spirochetes with variations in "protective" Osps -- and that these variant spirochetes preferentially live in patients and mice with chronic infection. The experiments will yield information on the factors that influence B. burgdorferi persistence and the pathogenesis of Lyme disease.